# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: The development of bubble sextants**

**From:**Frank Reed

**Date:**2009 Aug 15, 08:26 -0700

Quoting Gary: "Coriolis is a fictitious force use to explain the perceived curve of the flight path as observed by an observer on earth of other rotating frame of reference." Coriolis acceleration is a "frame-dependent" acceleration. That is, it depends on the choice of coordinates that you use to define motion. The expression "fictitious force" is a technical term intended to describe this frame-dependence. Just so there's no misunderstanding, it DOES NOT mean that the Coriolis acceleration is "fictional" in the ordinary sense of the English word. Of course, Coriolis acceleration is "real" in the sense that in many circumstances you find that the natural coordinates to describe the problem are rotating coordinates. The weather on Earth or any other planet, for that matter, is nearly impossible to understand in non-rotating coordinates but quite easy to understand in coordinates which rotate. From a modern perspective (modern = since the early part of the 20th century, after the development of general relativity), even the common acceleration of gravity near the surface of the Earth (at a single point) at 9.8 m/s^2 or 32 ft/sec^2 is a "fictitious force" since it can be eliminated by going to a frame of reference which is accelerating toward the Earth's center, in other words a freefall frame. And sure enough, if you place an aircraft on a parabolic trajectory with that acceleration, gravity disappears and passengers are rendered completely weightless exactly as if they are in orbit (until the plane's acceleration trajectory is changed). It is not "as if" there is no gravity; in that frame of reference, there really is "no gravity" apart from local tidal accelerations. Many people are able to explain the origin of the Coriolis acceleration by describing how it appears in an inertial, non-rotating frame of reference. In such a frame, the object moves on a straight line "while the Earth turns beneath it". This is very important information, of course, but it is a derivation, like an "etymology". It tells us why this acceleration must exist in non-inertial frames of reference, but it doesn't mean that it is fake or "fictional". And you surely wouldn't want to revert to the derivation once you understand why it works. In many ways, the expression "fictitious force" in physics has caused as many problems as the expression "imaginary number" in mathematics. Neither of these concepts are invalid or fictional or "un-real". For modern mathematics, so-called imaginary numbers have no defects, no lack of reality to them. They are what they are: solutions to algebraic equations. Likewise in physics, so-called fictitious forces are not forces to be avoided or treated as "un-real". They are what they are: forces arising from the choice of coordinates. -FER --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To unsubscribe, email NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---